The present invention relates to signal sensing for the purpose of detecting a special pattern of signals, and more particularly to utilization of a programmed processor for sensing and detecting a special pattern.
In magnetic recording, and in particular magnetic recording on disc files, a double frequency recording technique is utilized. Double frequency recording involves the recording of magnetic flux transitions at a clock frequency, with binary data being represented by the presence or absence of a flux transition between clock flux transitions. This form of recording provides a means by which a variable frequency oscillator (VFO) can be made responsive to reproduced signals from the magnetic recording to synchronize and control the sampling of the recorded data.
In magnetic disc recordings, each track is normally divided into a number of sectors, with each sector being comprised of various identifying and control characters for the purpose of data detection within the sector. One item of control information for the purpose of controlling further reproduction, is a field called an address mark. The fixed data content of an address mark field identifies the type of field which follows the address mark. The file control unit is caused to search for this particular data combination to control the use of the subsequent identified information. Since other portions of the recorded information may include normal data having the same data content as the address mark field being sought, a further form of signal detection has been incorporated into the detection of the particular data field. Address marks incorporate a special form of magnetic recording in which the binary data is interspersed with normal clock periods. However, instead of there being a magnetic flux transition for each clock period, certain flux transitions at clock times are eliminated from the recording. Therefore, a magnetic disc file control unit will recognize, with certainty, detection of the address mark, when the unique pattern of data is detected, along with the unique pattern of clocks and missing clocks within the eight-bit character being sensed.
Because of the high frequency at which the data and clock signals must be detected, previous file control units have required special purpose hardware and logic utilized only for this purpose. Even when magnetic disc file control units have been implemented utilizing programmed mini or microprocessors, the special purpose hardware has had to be included because the processors did not have the speed and instruction power necessary to operate at the high speed.
It is therefore the object of the present invention to provide a program controlled microprocessor, with simple circuitry, suitable for detecting the data and clock sequence of address marks.